System for phase inversion ultrasonic imaging

ABSTRACT

A system and method for ultrasonic imaging utilizing multiple sets of transmit pulses differing in amplitude, frequency, phase, and/or pulse width. One embodiment has phase differences between the k transmit signal asdegrees providing for constructive interference of the k&lt;th &gt;order harmonic pulse, while an amplitude modulation of each transmit profile is constant between sets. These sets of pulses are transmitted into media of interest and received echoes from these pulses are combined to form an averaged signal. The averaged pulses represent the net common mode signal received from each of the transmit sets. This combined signal set is used to reconstruct an ultrasound image based on broad beam reconstruction methodology.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ultrasound imaging, and moreparticularly, to a method and apparatus for improving and enhancingultrasound images.

2. Description of the Background Art

Ultrasonic imaging is frequently used for a variety of diagnosticprocedures due to its non-invasive nature, low cost, and fast responsetime. These qualities are especially true in medical fields where theadded benefit is reducing or eliminating a patient's exposure toradiation. Typically, ultrasound imaging is accomplished by 1)generating and directing an ultrasonic beam into media underinvestigation; and 2) observing any resulting waves that are reflectedback from dissimilar tissues and tissue boundaries within that area. Theresulting waves are received as signals. These received signals are thenpost-processed and imaged on a screen by plotting a spot whose intensityis proportional to the amplitude of a reflected beam from a givenlocation. Determination of location is based upon a known transmissionand re-radiation rate after the ultrasonic wave is pulsed into the mediaunder investigation.

Typically, an ultrasonic signal transmitted into the media underinvestigation includes a burst of sinusoidal waves of a given waveform.These sinusoidal waves are applied to a transducer and form atransmitted signal. The transmitted signal is typically in the range of40 kHz to 50 MHz, but more commonly, in the range of 40 kHz to 1 MHz. Asthe transmitted signal interacts with tissue layers and boundariesbetween layers, the ultrasonic signal is modified by being scattered,resonated, attenuated, reflected, or transmitted.

Media under investigation are often a non-linear media such as thosecommonly found in the human body. Non-linear media produce harmonicfrequencies in the echoed signals. These additional frequency componentscontinue to re-radiate through and, in turn, reflect off or interactwith other structures. A portion of the reflected (or echoed) signalspropagates back to a receiving transducer.

Fundamental and harmonic frequencies that are impinged upon a receivingtransducer includes the full signal, which must then be furtherprocessed to eliminate noise and extraneous components. The receivingtransducer may be the same as a transmitting transducer, or can becompletely independent. When the same transducer is used, atransmit/receive (T/R) switch connects the transducer to either thetransmitter electronics or the receiver post-processing electronics. Thereceiving transducer accepts the echo signal plus any generated noiseand furnishes these to a portion of the post-processing electronicsknown as a beam former. Beam formers reject noise and have either anadaptive or fixed configuration. Adaptive beam formers are designed toreject variable directional noise sources by monitoring the noise fieldand adjusting internal parameters to minimize the background noise.Fixed beam formers are designed to reject isotropic noise and takeadvantage of the directional property of the reflected signal.

Ultimately, ultrasonic images of the human body are a product ofharmonic imaging. Harmonic imaging is generally associated with eithervisualization of tissue boundaries and densities of different media, orimaging contrast agents at harmonic frequencies. Contrast agents aretypically fluid filled micro-spheres that resonate at ultrasonicfrequencies. Such agents are injected into the blood stream and arecarried to various parts of the body. Once these agents are pulsed atultrasonic frequencies, harmonic echo-locator signals are generated dueto the resonance produced within the micro-spheres.

While ultrasonic procedures have a distinct number of advantages overother types of diagnostic techniques, prior art methods and systems havenoise problems that make it difficult to determine the exact locationand proper interpretation of the received signal. Various forms ofaveraging techniques have been employed to reduce the noise, butaveraging alone is ineffective in locating images of interest betweentissues with similar densities. (Echoed signals from tissues withsimilar densities will indicate a uniform mass with indistinctboundaries. Averaging won't help in this situation.) Theseinterpretation difficulties are exacerbated by the fact that manytissues in the human body have similar densities. Therefore, a methodand system are needed that can effectively overcome the stateddifficulties while not negating the positive benefits of ultrasoundimaging systems in general.

SUMMARY OF THE INVENTION

The present invention is a system and method for generating enhancedultrasonic images. The invention utilizes multiple ultrasonic pulsesthat are transmitted in an alternating fashion into media of interest.These media being imaged may be a human body or some other linear and/ornon-linear media. The ultrasonic pulses are modulated in a way that mayvary in amplitude, frequency, phase, or pulse width. Each set ofultrasonic pulses is out-of-phase with other ultrasonic pulses by$\frac{360{^\circ}}{k},$where k is the number of pulse sets in the pulse sequence for a giventransducer element number, n. An out-of-phase condition is a propertywhen waveforms are of the same frequency but do not have correspondingintensity values at the same instant. The echo signals generated by thenon-linear media interacting with these out-of-phase signals aremeasured and appropriately combined.

The present invention is based on the observation that many types ofmedia scatter sound in a non-linear manner. With an ultrasonic imagingsystem based on linear-scattering media, the return signal is atime-shifted, amplitude-scaled version of the incident signal.Non-linear scattering media produce signals that cannot be produced bysimple time-shifts, scaling, or summation of the signal incident to ascattering site. The phase of an ultrasound wave reflected from theboundary of a non-linear medium is altered in a manner that depends onthe phase of the incident sound pulse. For example, consider the specialcase of two ultrasound pulses (k=2), where the phase difference betweenthe two transmitted sound pulses differ by$\frac{360{^\circ}}{k} = \frac{360{^\circ}}{2}$or 180 degrees. If the scattering site were purely linear then thereceived signal from each of the transmitted pulses would be the inverseof each other. These inverse signals, when averaged, would have a sum ofzero. If, however, there are signals generated from a non-linear processwithin the media, then these signals will not be the inverse of eachother and, consequently, will not sum to zero.

This non-linear property can be used to construct a system that looks atthe non-linear regions within given media. For example, in oneembodiment of this system, an image area is formed from three differentsets of transmitted signals each differing in phase by 120 degrees. Thelinear reflections generated by these k=3 sets of excitation pulses willcancel each other, while the k^(th) order non-linear components willnot. This pulse cancellation allows an averaged set of raw receiveddata, F(n, t), to be produced. A three-dimensional averaged set of rawreceived data, F(n,m,t), may also be produced where m is an element froma two-dimensional, or n×m transducer array. The data functionality isdependent on the channel (or transducer) number and time. This singleset of averaged data can then be used to re-construct an image area. Theimage area reconstructed would represent the information generated fromthe third, or k^(th), harmonic, generated from the scattering siteswithin the media.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic block diagram of an embodiment of an ultrasoundimaging system using the present invention;

FIG. 2 shows an exemplary modification of one of a potential pluralityof waveforms being modified;

FIG. 3 shows an embodiment of a signal transmitter unit prior to signaldelivery to media of interest;

FIG. 4A shows an embodiment of a receiver and raw data averager unit;

FIG. 4B shows an alternative embodiment of a receiver and raw dataaverager unit; and

FIG. 5 shows an embodiment of a data processing unit.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the field of ultrasound imaging. Thisinvention utilizes broad beam technology (B² Technology™) to performimage extraction of the non-linear elements of media underinvestigation. These media will hereinafter be referred to as media ofinterest. Broad beam technology defines an area under investigation at agiven point in time, which is in contrast with a system utilizing afocused beam.

FIG. 1 is a block diagram of an embodiment of an ultrasound imagingsystem using the present invention. Imaging system 100 includes at leastone signal generator unit 110, at least one signal transmitter unit 120,media of interest 130 to be imaged, at least one receiver and raw dataaverager unit 140 to capture signals received from the media of interest130, and a data processing unit 150 for taking the averaged receivedsignals and producing an area of image formation on an image displayunit 160.

A signal generator unit 110 drives circuitry for a signal transmitterunit 120. The signal transmitter unit is shown and described in greaterdetail in FIG. 3.

A signal transmitter unit 120 transmits pulsed sets of ultrasonic energyinto the media of interest 130. Echoes received from the media ofinterest 130 are stored in the receiver and raw data averager unit 140.Subsequent out-of-phase signals from the signal generator unit 110 passthrough the signal transmitter unit 120, and are converted into pulsedsets of ultrasonic energy that travel to the media of interest 130. Themedia of interest 130 modifies the pulsed sets of ultrasonic energy.These modified pulsed sets of ultrasonic energy are received andaveraged by the receiver and raw data averager unit 140. Data from thesereceived pulsed sets are averaged in a data set as a function of channelnumber, n, and time, t. The averaged data sets are processed by the dataprocessing unit 150 and displayed on an image display unit 160.

FIG. 2 shows an exemplary modification of one of a potential pluralityof waveforms being modified. This FIG. 2 example demonstrates how asignal generator unit 110 may modify a generated signal. For example, astated modulation may be in the form of any combination of varying anamplitude, frequency, or pulse width of an unaffected input signal.These modulated signals will additionally vary in phase for a givenpulse set. To produce a modulation, an envelope function, A(n,t), may beconvolved with a sine wave, depicted by e^(j[{overscore (ω)}) ^(o)^((t,n)t+θ) ^(i) ^(+θ(n)]) giving the final waveform A(n,t)e^(j[{overscore (ω)}) ^(o) ^((t,n)t+θ) ^(i) ^(+θ(n)]). In this waveformnotation, n is the transducer element number, and i is a given pulseindex (e.g., if a second harmonic is utilized, k=2, then i=1 . . . 2).The phase varies for different pulse sequences within a given pulse setand is indicated by the θ_(i) notation.

To illustrate the concept of phase variation, take an example where k isthree. In this example, each pulse within a pulse set is varied in phaseby $\frac{360{^\circ}}{k} \equiv \frac{360{^\circ}}{3}$or 120°. A first pulse is generated with a 0° phase orientation, asecond pulse is 120° out-of-phase with the first pulse, and a last pulsein the pulse set is 240° out-of-phase with respect to the first pulse.After the first pulse is transmitted and received, the second pulse istransmitted and received, and so on through the sequence. Allinformation is tracked so that fundamental frequencies can be summed andeliminated, leaving primarily only harmonically generated echoes. Recallthat harmonically generated echoes are produced by non-linear media.

Further, as an example, an envelope function, A(n,t), may be a Gaussianwaveform. The transmitted signal may additionally be modulated as achirped waveform (i.e., swept-frequency modulation, a Fourier transformof which is still centered around the fundamental with a broaderdispersion). Optionally, a digital waveform generator could be used inplace of the convolution method shown in FIG. 2.

FIG. 3 shows an embodiment of a signal transmitter unit 120 prior tosignal delivery to media of interest. The signal transmitter unit 120includes at least a power amplifier 330, a transmit/receive switch 340,and a first transducer 350. Optionally, a signal transmitter unit 120may further include a delay circuit 310. The delay circuit 310 may be ananalog or digital delay. Also, optionally, the signal transmitter unit120 may include a channel gain unit 320 to drive the power amplifier 330as a function of channel number and time. Additionally, the signal orpulse may be pulse-width modulated (not shown) to conserve power. Powerconservation can become crucial in field applications of the systemwhere battery power may be utilized.

FIG. 4A shows one embodiment of a receiver and raw data averager unit140. A second transducer 410 receives pulsed sets modified by media ofinterest 130. These received pulsed sets are transformed from ultrasonicenergy into an electrical signal by the second transducer 410. A secondtransmit/receive switch 420 may be used to couple the electrical signalsto the appropriate circuitry. In one embodiment, a second transducer 410and a second transmit/receive switch 420 may be coincident or analogousunits to the first transducer 350 and first transmit/receive switch 340shown in FIG. 3. A second power amplifier 430 may be added andcontrolled as a function of time by gain control unit 440. The output ofpower amplifier 430 sends an amplified signal to an optional bandpassfilter 450. The bandpass filter 450 may be used, among other things, toreduce or eliminate extraneous noise. FIGS. 4A and 4B share the samecomponent layout up to and including bandpass filter 450, wherethereafter they diverge thereby exemplifying alternate embodiments.

The electrical signal of the FIG. 4A embodiment is coupled to a firstanalog-to-digital (A/D) converter 460, and may continue into an optionalin-phase and quadrature (I/Q) mixer 470, which produces a singleside-band signal, optional first baseband filter 480, and to an averager490. The optional first baseband filter acts to reduce or eliminate anyfundamental frequency from the signals received from the original pulsesets, leaving primarily harmonically generated signals. One purpose ofthe averager is to provide a point-by-point arithmetic average of thereceived electrical signals. Mathematically, this arithmetic average maybe expressed as${{F\left( {n,t} \right)} = {\sum\limits_{i = 1}^{k}\frac{R_{i}\left( {n,t} \right)}{k}}},$where the received signal, R_(i), is summed for each element of thetransmit cycle as a function of channel number and time to correlatewith the original transmitted pulse, i. All other components in thesignal path are of types commonly known to one of ordinary skill in theart.

FIG. 4B shows an alternative embodiment of a receiver and raw dataaverager unit. Recall FIGS. 4A and 4B share the same component layoutthrough and including the optional bandpass filter 450. From the pointof this optional bandpass filter 450, the signal of the FIG. 4Bembodiment is further coupled to an analog mixer 455, an optional secondbaseband filter 465, a second analog-to-digital converter 475, and anaverager 490.

FIG. 5 shows an embodiment of the data processing unit 150. Here, dataprocessing unit 150 receives averaged data from the receiver and rawdata averager unit 140. The averaged data are input to the dataprocessing unit 150 and received at I/Q raw data matrix 510, whichstores the averaged data in an M×N area array, where M is the number ofsamples (1 to 10,000 samples is an exemplary number) and N is the numberof elements×2 (both in-phase and quadrature). These averaged data arefed into a digital signal processor (DSP) 520, which reconstructs theraw data into an area of acoustic image. An exemplary reconstructionequation may take the form of${I\left( {r,\varphi} \right)} = {\sum\limits_{i = 1}^{k}{{a_{i}\left( {r,\varphi} \right)} \cdot {\mathbb{e}}^{{j\theta}_{i}{({r,\varphi})}} \cdot {{F\left\lbrack {i,{t_{i}\left( {r,\varphi} \right)}} \right\rbrack}.}}}$In this equation a, indicates an aperture function, r refers to a radialdistance from a transducer center at a given angle φ, and the function Fis an averaged set of raw received data. The digital signal processor520 functions could be achieved in any number of ways, For example, inan alternative embodiment, a properly designed application-specificintegrated circuit (ASIC) could be used in place of the digital signalprocessor 520. These converted data in polar coordinates are saved in anacoustic image data buffer 530 in a J×K matrix (where J is the number ofrange samples and K is the number of angular samples). At this point,the data are still a function of a distance, r, from the transducer at agiven angle, φ. This could also be accomplished in a Cartesiancoordinate system. The acoustic image data buffer 530 allows the data tobe stored until needed by scan converter 540. The I(r, φ) image data areconverted into a reconstructed image in Cartesian coordinate data I(x,y)through the use of an r-φ scan converter 540. An r-φ scan converter iswell known in the art and typically converts two-dimensional data frompolar to Cartesian coordinates by means of the conversion, x=r cos(φ)and y=r sin(φ).

Output from the data processing unit 150 produces an image area I(x,y)corresponding to an area irradiated by pulsed sets of ultrasonic energy.These converted I(x,y) data may be displayed on image display unit 160.Image display unit 160 may be any visual display such as, but notlimited to, a computer monitor, flat-panel or liquid-crystal display,cathode-ray tube (CRT), or the like.

From the description of the preferred embodiments of the process andapparatus set forth supra, it will be apparent to one of ordinary skillin the art that variations and additions to the embodiments can be madewithout departing from the principles of the present invention. Forexample, it could be easy to envision a system whereby an entirethree-dimensional (3D) volume could be displayed at once as opposed to atwo-dimensional area. This three-dimensional embodiment may beaccomplished by holography or some other means. It would be an obviousextrapolation from the tenets of the two-dimensional system presentedherein to construct a three-dimensional apparatus.

1. A system for ultrasonic imaging, comprising: a signal generator unitfor generating at least two out-of-phase pulses, wherein said signalgenerator unit may modulate at least two sine waves with an envelopefunction to produce a modified version of said at least two out-of-phasepulses, said signal generator unit further converting said at least twoout-of-phase pulses into at least two out-of-phase acoustical pulses; asignal transmitter unit coupled to said signal generator unit fortransmitting said at least two out-of-phase acoustical pulses into mediaof interest; a receiver and raw data averager unit for receiving said atleast two acoustical pulses modified by said media of interest, whereindata from said at least two modified acoustical pulses are averaged in adata set as a function of channel number and time; and a data processingunit coupled to said receiver and raw data averager unit, wherein saiddata processing unit constructs image data from said modified acousticalpulses, an output from said data processing unit is further coupled toan image display unit.
 2. The system of claim 1, wherein said signaltransmitter unit comprises a power amplifier, a transmit/receive switch,and a transducer.
 3. The system of claim 2, further comprising a digitaldelay circuit.
 4. The system of claim 2, further comprising an analogdelay circuit.
 5. The system of claim 2, further comprising a channelgain circuit.
 6. The system of claim 1, wherein said receiver and rawdata averager unit comprises a transducer, a transmit/receive switch, ananalog-to-digital converter, and an averager.
 7. The system of claim 6,wherein said receiver and raw data averager unit further comprises apower amplifier, a bandpass filter, and a baseband filter.
 8. The systemof claim 6, wherein said receiver and raw data averager unit furthercomprises an in-phase and quadrature mixer.
 9. The system of claim 1,wherein said signal generator unit and said receiver and raw dataaverager unit share a transducer.
 10. The system of claim 1, whereinsaid data processing unit comprises an in-phase and quadrature mixer, adigital signal processor, an acoustic image data buffer, and a scanconverter.
 11. The system of claim 1, wherein said data processing unitcomprises an in-phase and quadrature mixer, an application specificintegrated circuit, an acoustic image data buffer, and a scan converter.